The present invention relates to a light alloy piston for internal combustion engines, which piston comprises a piston skirt that is closed at its lower end and, at locations disposed adjacent to the bosses on both sides of the horizontal plane extending through the piston pin, is inwardly offset from the outside peripheral surface of the piston to such an extent that the distance between the outer ends of the bosses amounts to 60 to 80% and the overall width of the load-carrying portion of the skirt amounts to 70 to 55% of the piston diameter.
In view of the high requirements to be met by modern high-speed internal combustion engines as regards their running properties, it is desired in the art, inter alia, to reduce the weight of components of internal combustion engines because this will reduce the fuel consumption so that energy can be saved. The reduction in weight also has a favorable influence on the behavior of the engine as regards noise and vibration so that desired high riding comfort can be obtained. Part of these objects of development can be achieved by the use of lightweight light alloy pistons because they constitute smaller oscillating masses. On the other hand, a reduction in weight must not adversely affect the safety of operation and the life of the internal combustion engine.
It is known that modern methods of investigation, such as the finite elements method, permit a determination of the strength limits of conventional solid-skirt light alloy pistons having minimum piston skirt and piston head thicknesses. The result of these investigations is the light alloy piston having a reduced weight. The reduction of all wall thicknesses and of the interior profiles have permitted a reduction in weight by about 10%. Further developments have resulted in the provision of a light alloy piston in which the weight has been reduced where the piston skirt is provided, adjacent to the bosses on both sides of the horizontal plane extending through the piston pin, with respective recesses formed by inwardly offset, straight or arcuate portions of the piston skirt. The bosses are offset toward the center as far as possible so that the distance between the outer ends of the bosses is not in excess of 65% of the diameter of the piston and a very short piston pin can be inserted, which has an overall length of 60% of the piston diameter. The distance between the inner ends of the bosses amounts only to 25% and the width of the bosses amounts only to 22% of the piston diameter. The shortening of the piston pin and the reduction of the width of the bosses has resulted in a distinct reduction in weight by up to 20% of the weight of the conventional solid-skirt piston. The circular shape of the skirt is retained at its end so that the guidance of the piston along a straight line is ensured. In spite of its light weight, that piston has the same satisfactory performance and strength characteristics as a conventional solid-skirt piston under all operating conditions. A similar behavior is exhibited by the light alloy piston which is described in laid-open German application No. 2841980 and comprises a piston skirt which, adjacent to the piston pin bosses, is inwardly offset from the outside peripheral surface of the piston and which skirt is non-symmetrical with respect to the vertical plane through the piston pin. In that piston, the piston skirt is connected to the piston pin bosses by carrying ribs, which extend at an angle of 30 to 60 degrees to the piston pin axis on one side and at an angle of 90 degrees to said axis on the other side.
It has been found, however, that an excessive quantity of oil is collected in the recesses of the light alloy piston described first hereinbefore under certain operating conditions, in dependence on the overall oil balance of the internal combustion engine, and that that quantity of oil cannot flow off as fast as is required. During the operation of the engine that quantity of oil will adversely affect the return flow of the oil that has been scraped from the cylinder through the oil scraper ring disposed in the lower ring-carrying zone so that oil may be retained and may be forced past the ring-carrying zone into the combustion chamber, wherein that oil is burnt so that the oil consumption of the internal combustion engine is increased.